A copolymerization reaction between a compound having an ethylenic carbon-carbon double bond (hereinafter referred to as the “enic compound”) and a thiol compound (hereinafter referred to as the “ene-thiol reaction”) and a copolymer thereof are useful techniques well known in the art. The ene-thiol reaction is progressed by light and heat even if no polymerization initiator is used, but as a practical curing method, a photo-curing method or thermal curing method using a radical polymerization initiator is widely used. The ene-thiol reaction has characteristics such as a high reaction rate, little influence of oxygen inhibition and low cure shrinkage. For this reason, intended use as a rapid curing resin such as a coating material, a seal material, an encapsulation material and an adhesive, intended use for a molded product such as an optical resin, etc. have been proposed (Patent Document 1, Non-patent Document 1, etc.). Particularly, a polymerizable composition consisting of an enic compound and a thiol compound having a high refractive index has been proposed to be applied to an optical material having a high refractive index (Patent Documents 2, 3, etc.).
When a polymerizable composition comprising a polythiol compound and an enic compound having a phenylthio backbone is polymerized and cured, a cured product obtained has a particularly high refractive index, and therefore such a polymerizable composition is useful as an optical material. Patent Document 4 reports a polymerizable composition having a high refractive index using 4,4′-bis(methacryloylthio)diphenylsulfide (hereinafter referred to as “MPSMA”) which is a representative phenylthio(meth)acrylate compound. However, a composition obtained by simply dissolving an enic compound having a phenylthio backbone in thiol is unstable even in a cool dark place, a thermal polymerization reaction thereof proceeds and the viscosity increases, and therefore, there is a practical problem that such a composition cannot be preserved for a long period of time. For this reason, the development of a method for stably preserving a polymerizable composition comprising a thiol compound and an enic compound having a phenylthio backbone had been desired.
During use of an ene-thiol composition, temporal stability thereof is very important, and various stabilization methods such as reduction of acid value, a metal ion and an inorganic ion (Patent Documents 5-7) and addition of a stabilizer such as nitrone-based, nitroxide-based and nitroso-based compounds (Patent Documents 8 and 9), a quinone-based compound (Patent Document 10), an iodine-based compound (Patent Document 11) and a phosphorus-based compound (Patent Document 12) have been reported. However, the reactivity of the ene-thiol composition significantly varies depending on the molecular structure of a monomer, particularly an enic compound, and the effective stabilizer also varies, and therefore, it is difficult to infer the effect of a stabilizer from already-known reported examples.